Circuit configuration with a controllable current limiting circuit for driving a load

ABSTRACT

A circuit configuration for driving a load is described. The circuit configuration has a first and a second connecting terminal for connecting the load, a first drive input for applying a first drive signal, and a first semiconductor switching element having a first load terminal connected to the first connecting terminal, a second load terminal connected to the second connecting terminal and a drive terminal coupled to the drive input. A voltage limiting circuit is provided and is connected between the first load terminal and the drive terminal of the first semiconductor switching element.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a circuit configuration for driving aload. The circuit configuration contains a first connecting terminal forconnecting to the load, a second connecting terminal, a first driveinput for receiving a first drive signal, and a semiconductor switchingelement having a first load terminal connected to the first connectingterminal, a second load terminal connected to the second connectingterminal, and a drive terminal coupled to the first drive input. Avoltage limiting circuit is connected between the first load terminaland the drive terminal of the semiconductor switching element.

U.S. Pat. No. 4,658,203 discloses a circuit configuration for driving amotor, which has a power MOSFET whose load path (drain-source path) isconnected between a connecting terminal of the motor and ground. In thiscase, the gate terminal of the MOSFET is connected to a pulse widthmodulator for driving the MOSFET. In order to limit the voltage acrossthe load path of the MOSFET, a reverse-biased zener diode is connectedbetween the drain terminal and the gate terminal of the MOSFET. If thepotential at the drain terminal of the MOSFET exceeds a predeterminedvalue which is dependent on the breakdown voltage of the zener diode,then the zener diode turns on and charges the gate capacitance of theMOSFET, as a result of which the MOSFET turns on and as a result ofwhich the voltage across the load path thereof is limited. In this case,the zener diode is dimensioned in such a way that it turns on in orderto drive the MOSFET before the breakdown voltage of the transistor isreached.

For switching loads it is known, moreover, to use so-called smart powerFETs. Components of this type contain, in addition to a powertransistor, inter alia a protective circuit for the power transistorthat is intended to protect the power transistor against, for example,an excessively large load current or an excessively high temperature.The protective circuit usually has a switch which is connected to thegate terminal of the power transistor and serves for discharging thegate capacitance of the transistor in order to turn off the transistorif, for example, the load current of the transistor or the temperaturethereof assumes a value at which there is the risk of the transistorbeing damaged.

If a power transistor for driving a load contains a protective circuitwith a switch for discharging the gate capacitance and also a zenerdiode for voltage limiting, then the situation can arise wherein theswitch of the protective circuit turns on in order to turn off the powertransistor, and wherein, at the same time, the zener diode turns on inorder to drive the power transistor and thereby to limit the voltageacross the load path thereof. Whereas only a short current pulse flowsthrough the zener diode, in order to charge the gate capacitance, whenthe switch of the protective circuit is turned off, a current flowspermanently through the zener diode when the switch of the protectivecircuit is turned on and the zener diode is turned on. Due to anunavoidable internal resistance of the zener diode or of another voltagelimiting circuit, which may be not inconsiderable particularly in thecase of integrated zener diodes, there is then the risk that, on accountof the voltage drop which is additionally brought about across the zenerdiode and results from the product of the flowing current and theinternal resistance, the drain potential of the power transistor willrise to a value at which the power transistor is in danger ofdestruction.

In addition, the MOSFET turns off rapidly if its gate capacitance isdischarged by the protective circuit. In this case, inductive loads orelse only the inductance of the leads can give rise to high inducedvoltages in the leads which can reach the level of the breakdown voltageof the MOSFET.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a circuitconfiguration with a controllable current limiting circuit for driving aload which overcomes the above-mentioned disadvantages of the prior artdevices of this general type, in which it is possible to use asemiconductor switching element with a protective circuit, in particularwith a protective circuit against an over temperature or for currentlimiting, and wherein the semiconductor switching element isadditionally protected against an over-voltage on its load path.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a circuit configuration for driving aload. The circuit configuration has a first connecting terminal forconnecting to the load, a second connecting terminal, a first driveinput for receiving a first drive signal, and a semiconductor switchingelement having a first load terminal connected to the first connectingterminal, a second load terminal connected to the second connectingterminal, and a drive terminal coupled to the first drive input. Avoltage limiting circuit is connected between the first load terminaland the drive terminal of the semiconductor switching element. Thevoltage limiting circuit has a second drive input receiving a seconddrive signal, and a threshold voltage of the voltage limiting circuit isdependent on the second drive signal.

The second drive signal may be variable for example depending on theswitching state of the first semiconductor switching element, atemperature in the region of the first semiconductor switching elementor a current through the first semiconductor switching element. Thevoltage limiting circuit is configured in such a way that a thresholdvoltage of the voltage limiting circuit is reduced in the case of thosestates of the first semiconductor switching element in which the voltagelimiting circuit can permanently take up a current or in which it ispossible for the semiconductor switch to be switched off rapidly by aprotective circuit. This prevents the situation in which, as a result ofa rise in the voltage across the voltage limiting circuit which resultsfrom a voltage drop brought about by a permanent current at an internalresistance of the voltage limiting circuit, the potential at the firstload terminal of the first semiconductor switching element rises to avalue at which the first semiconductor switching element is in danger ofdestruction. A situation in which the voltage limiting circuitpermanently takes up current can arise when a protective circuit ispresent which short-circuits the drive terminal and the second loadterminal of the first semiconductor switching element in the overloadcase, in order to turn off the first semiconductor switching element.

If the switch is in a state in which a rapid switch-off by a protectivecircuit can occur, then the threshold voltage of the voltage limitingcircuit is preferably likewise reduced. In this case, the voltagelimiting circuit counteracts the protective circuit and, uponcommencement of the protective circuit, from the point when the reducedthreshold voltage is reached, prevents the gate capacitance from beingdischarged too rapidly and thereby prevents high induced voltages on theload path of the MOSFET.

One embodiment of the circuit configuration according to the inventionprovides for the voltage limiting circuit to have a series circuitcontaining a first and a second voltage limiting element, it beingpossible for one of the voltage limiting elements to be short-circuitedaccording to the second drive signal. In order to short-circuit the onevoltage limiting element, in one embodiment of the invention, a switch,in particular a semiconductor switching element, is connected inparallel with the voltage limiting element, the switch having a driveterminal to which the second drive signal is fed. The series-connectedvoltage limiting elements are preferably zener diodes, which have theadvantage that they can be integrated in a simple manner in the samesemiconductor body as the first semiconductor switching element.

Depending on the switch position of the second switch, the voltagelimiting circuit according to the invention has a breakdown voltage thatcorresponds to the sum of the breakdown voltages of the two voltagelimiting elements or the breakdown voltage of the voltage limitingelement which cannot be short-circuited.

In one embodiment of the invention, the second drive signal is dependenton a switching state of the first semiconductor switching element. As aresult, a high breakdown voltage of the voltage limiting circuit can beset in the case of a first switching state, in which the firstsemiconductor switching element turns off, whereas a lower breakdownvoltage of the voltage limiting circuit is set in the case of a secondswitching state, in which the first semiconductor switching elementturns on. In the case of the switching state mentioned last, given thepresence of a protective circuit which serves for turning off the firstsemiconductor switching element in an overload case, the situation canarise wherein a current permanently flows through the voltage limitingcircuit, since part of the current through the voltage limiting circuitwhich is intended to turn on the first semiconductor switching elementis taken up by the protective circuit. The protective circuit is used toachieve an opposite aim, namely to turn off the first semiconductorswitching element. Due to an internal resistance that is inevitablypresent in the voltage limiting circuit, in the event of a permanentcurrent flow through the voltage limiting circuit, the voltage acrossthe voltage limiting circuit rises. The voltage limiting circuit has abreakdown voltage that is reduced in this case, however, and thisprevents the voltage across the load path of the first semiconductorswitching element from rising to a value at which the firstsemiconductor switching element is in danger of destruction.

One embodiment of the invention provides a protective circuit with athird semiconductor switching element, the third semiconductor switchingelement is connected between the control terminal of the firstsemiconductor switching element and the second load path terminalthereof. The third semiconductor switching element serves for turningoff the first semiconductor switching element according to a drivesignal, in that it short-circuits the drive terminal and the second loadpath terminal. In this case, the drive signal of the third semiconductorswitching element is preferably dependent on a temperature in the regionof the first semiconductor switching element and/or on a load current ofthe first semiconductor switching element.

A second embodiment of the invention provides for the second drivesignal to be dependent on a temperature in the region of the firstsemiconductor switching element. As a result, the breakdown voltage ofthe voltage limiting circuit can be reduced for example when thetemperature in the region of the first semiconductor switching elementexceeds a predetermined value at which a protective circuit reacts inorder to turn off the first semiconductor switching element. In thiscase, too, the protective circuit pursues an opposite aim to that of thevoltage limiting circuit, namely the aim of turning off the firstsemiconductor switching element, which results in a permanent currentflow through the voltage limiting circuit. The reduced breakdown voltageof the voltage limiting circuit in this case prevents the breakdownvoltage of the first semiconductor switching element from being reached,the first semiconductor element being in danger of destruction at thebreakdown voltage.

In a corresponding manner, a further embodiment of the inventionprovides for the second drive signal to be dependent on a currentthrough the first semiconductor switching element. In this case, if thecurrent through the first semiconductor switching element exceeds avalue at which a protective circuit switches on for the purpose ofturning off the first semiconductor switching element, then, as in theover-temperature case described above, the breakdown voltage of thevoltage limiting circuit is reduced in order to intercept the voltagerise across the voltage limiting circuit, the voltage rise being broughtabout in the event of a permanent current flow, and to prevent thebreakdown voltage of the first semiconductor switching element frombeing reached.

The voltage limiting circuit preferably has a drive circuit for drivingthe second switch connected in parallel with the first voltage limitingelement, the drive circuit being fed a current signal which is dependenton the current through the first semiconductor switching element, and/ora temperature signal which is dependent on the temperature in the regionof the first semiconductor switching element. In this case, for theprovision of the current signal, a current sensor is connected in serieswith the first semiconductor switching element and, for the provision ofthe temperature signal, a temperature sensor is disposed in the regionof the first semiconductor switching element.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a circuit configuration with a controllable current limiting circuitfor driving a load, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a circuit configuration having a voltagelimiting circuit for driving a load in accordance with a firstembodiment of the invention;

FIG. 2 is a circuit diagram of the circuit configuration according tothe invention in accordance with a second embodiment, which has afurther protective circuit in addition to a voltage limiting circuit;

FIG. 3 is a circuit diagram of the circuit configuration according tothe invention in accordance with a third embodiment;

FIG. 4 is a circuit diagram of the circuit configuration according tothe invention in accordance with a fourth embodiment;

FIG. 5 is a circuit diagram of the circuit configuration according tothe invention in accordance with a fifth embodiment; and

FIG. 6 is a circuit diagram of the circuit diagram of a sixth exemplaryembodiment of the circuit configuration according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In all the figures of the drawing, sub-features and integral parts thatcorrespond to one another bear the same reference symbol in each case.Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a first exemplaryembodiment of a circuit configuration 10 according to the invention forconnecting a load Z to a supply potential V+, GND. The circuitconfiguration 10 has a first connecting terminal K1, which, in order toillustrate the function of the circuit configuration in FIG. 1, isconnected to the load Z. A terminal of the load Z that is remote fromthe connecting terminal K1 is connected to a first supply potential V+.The circuit configuration 10 has a second connecting terminal K2, whichis connected to a second supply potential GND in FIG. 1. In addition tothe first and second connecting terminals K1, K2, a drive input K3 ispresent, for feeding in a drive signal S1.

The circuit configuration 10 contains a first semiconductor switchingelement T1, which is configured as an n-channel power MOSFET in theexemplary embodiment. The transistor T1 has a drain terminal as a firstload path terminal, which is connected to the first connecting terminalK1, and the transistor T1 has a source terminal as a second load pathterminal, which is connected to the second connecting terminal K2. Agate terminal G as a drive terminal of the transistor T1 is connectedvia a resistor R1, which limits the drive current of the transistor T1,to the drive input K3 for feeding in the first drive signal S1. Thetransistor T1 has a gate-source capacitance, which is depicted as acapacitor Cg between a gate G and a source S in FIG. 1.

A voltage limiting circuit SB is connected between the drain terminal Dand the gate terminal G of the transistor T1 and, according to theinvention, is fed a second drive signal S2. The voltage limiting circuitSB is configured to drive the transistor T1 by charging the gate-sourcecapacitance Cg if a potential at the drain terminal D reaches a value atwhich there is the risk of the transistor T1 being destroyed. In thiscase, the transistor T1 is turned on in a manner driven by the voltagelimiting circuit SB, thereby preventing a further rise in the drainpotential or the load path voltage Uds of the transistor T1.

A voltage Udg present between the drain terminal D and the gate terminalG of the transistor is present across the voltage limiting circuit SB.The value of the voltage Udg, at which a threshold voltage of thevoltage limiting circuit SB is reached and the latter is activated inorder to drive the transistor T1, is variable and dependent on thesecond input signal S2. Preferably, at least two different thresholdvoltages can be set depending on the second drive signal S2.

FIG. 2 shows an exemplary embodiment of the invention in which thevoltage limiting circuit SB has a series circuit containing two voltagelimiting elements Z1, Z2, a first zener diode Z1 and a second zenerdiode Z2 in the exemplary embodiment. A second transistor T2, which is ap-channel MOSFET, is connected in parallel with the second zener diodeZ2, the second drive signal S2 is fed to a gate terminal G of the secondtransistor T2. The two zener diodes Z1, Z2 are connected in the reversedirection between the drain terminal D and the gate terminal G of thefirst transistor T1. A diode D1, which is connected in series with thefirst and second zener diodes Z1, Z2 and is connected in the forwarddirection between the drain terminal D and the gate terminal G of thefirst transistor T1, serves for preventing a current flow from the gateterminal G to the drain terminal D of the first transistor T1.

The circuit configuration 10 in accordance with FIG. 2 furthermore has aprotective circuit SC connected between the gate terminal G and thesource terminal S of the first transistor T1. The protective circuit SChas a third transistor T3, whose drain-source path D-S is connectedbetween the gate terminal G and the source terminal S of the firsttransistor T1. A third drive signal S3 from a drive circuit A3 of theprotective circuit SC is fed to a gate terminal G of the thirdtransistor T3. The drive circuit A3 is fed a temperature signal S4,which is dependent on a temperature in a region of the first transistorT1 and which, in the exemplary embodiment, is generated by a temperaturesensor TS disposed in a manner thermally coupled to the first transistorT1.

If, in the circuit configuration 10 in accordance with FIG. 2, thesecond transistor T2 is turned on by the second drive signal S2, thenthe second zener diode Z2 is short-circuited and the first transistor T1is turned on by the voltage limiting circuit SB when the drain potentialof the first transistor T1 exceeds the value of the gate potential ofthe first transistor T1 for instance by the value of the breakdownvoltage of the first zener diode Z1. If the second transistor T2 turnsoff in a manner driven by the second drive signal S2, then the firsttransistor T1 is turned on by the voltage limiting circuit SB if thedrain potential of the first transistor T1 exceeds the gate potentialfor instance by a value which corresponds to the sum of the breakdownvoltages of the first and second zener diodes Z1, Z2.

In this case, the switching state of the second transistor T2 ispreferably dependent on the switching state of the first transistor T1,the threshold voltage of the voltage limiting circuit SB at which thevoltage limiting circuit SB drives the first transistor T1 being lowerwhen the first transistor T1 is turned on than when the first transistorT1 is turned off.

If the first transistor T1 turns on and, in this case, an overtemperature occurs which is registered by the temperature sensor TS andat which the first transistor T1 is in danger of destruction, then thethird transistor T3 is turned on by the third drive signal S3 generatedby the drive circuit A3 in order to discharge the gate-sourcecapacitance Cg of the first transistor T1 and thereby to turn off thefirst transistor T1. As a result, the load path voltage Uds of the firsttransistor T1 rises, in which case, given the presence of an inductiveload Z or else due to the inductance of the leads to the transistor T1,the load path voltage Uds can rise considerably in a manner due toinduction. In this case, if the drain potential of the first transistorT1 reaches a value which exceeds the gate potential of the firsttransistor T1 by the value of the breakdown voltage of the first zenerdiode Z1 then a current flows from the drain terminal D via the secondtransistor T2, the first zener diode Z1 and the diode D1 to the gateterminal G of the first transistor T1, part of the current flowing awayto reference-ground potential GND via the turned-on third transistor T3.

The first zener diode Z1 unavoidably has an internal resistance which,in the event of a current permanently flowing through the first zenerdiode Z1, results in an additional voltage drop across the zener diodeZ1, which results from the product of the internal resistance and theflowing current. The additional voltage drop results in a rise in thedrain potential or the load path voltage Uds of the first transistor T1.In this case, the breakdown voltage Z1 of the zener diode is chosen suchthat a load path voltage Uds at which the first transistor T1 is indanger of destruction does not occur even as a result of the additionalvoltage rise caused by the internal resistance and the permanentcurrent.

In addition, the first zener diode Z1 counteracts a rapid discharge ofthe gate capacitance Cg in order to avoid high induced voltages that canbe caused by an inductive load or by the inductance of the leads. Inthis case, the breakdown voltage of the zener diode Z1 is dimensionedsuch that a further voltage rise in the load path voltage due toinductive effects does not lead to the breakdown voltage of thetransistor T1 being reached.

The zener diode Z1 and the third transistor T3 are coordinated with oneanother in such a way that when the third transistor T3 is turned on andthe threshold voltage of the voltage limiting circuit is reached, thecurrent taken up by the zener diode Z1 is greater than the current takenup by the third transistor T3.

If the first transistor T1 turns off, then it is usually not possiblefor an over temperature to occur at the first transistor T1, that is tosay the protective circuit for discharging the gate capacitance Cg isdeactivated. In this case, a current can flow only briefly via thevoltage limiting circuit SB in order to charge the gate capacitance Cg.In this case, the second switch T2 is open and the voltage limitingcircuit SB turns on when the drain potential exceeds the gate potentialby a value that corresponds to the sum of the breakdown voltages of thefirst and second zener diodes Z1, Z2. Since a permanent current flowthrough the voltage limiting circuit SB cannot take place in this case,there is no need to take account of an additional voltage rise due tothe permanent current flow and the internal resistance of the zenerdiodes Z1, Z2.

FIG. 3 shows an exemplary embodiment of the invention in which thesecond drive signal S2, which is fed to the p-conducting secondtransistor T2 in the exemplary embodiment, is formed by inversion of thefirst drive signal S1 by an inverter INV. In this embodiment of theinvention, it is ensured that the second transistor T2 turns on in orderto short-circuit the second zener diode Z2 if the first transistor T1turns on. The second transistor T2 turns off if the first transistor T1turns off. The drive circuit A3 of the third transistor T3 of theprotective circuit SC is fed a drive signal S5 which, in accordance withthe exemplary embodiment as shown in FIG. 2, may be a signal which isdependent on the temperature in the region of the first transistor T1,or else a signal which is dependent on a load current I of the firsttransistor T1.

FIG. 4 shows a further exemplary embodiment of the circuit configurationaccording to the invention, in which a delay element T is connectedupstream of the inverter INV. The second transistor T2 then switches ineach case in a time-delayed manner with respect to the first transistorT1.

FIG. 5 shows a further exemplary embodiment of the circuit configurationaccording to the invention, in which a drive circuit A2 is provided forgenerating the second drive signal S2 of the second transistor T2, whichdrive circuit A2, like the drive circuit A3 of the third transistor T3,is fed a temperature signal S4 dependent on the temperature in theregion of the first transistor T1. The second drive circuit A2 isconfigured to drive the second transistor T2 if the temperature signalS4 reaches a predetermined value, which is preferably less than thevalue at which the third switch T3 switches on. The breakdown voltage ofthe voltage limiting circuit SB is reduced in this way before the thirdswitch T3 is activated in the event of a further rise in temperature.

A further embodiment of the invention, which embodiment is notspecifically illustrated, provides for the second transistor T2 and thethird transistor T3 to be driven simultaneously, the second drive signalS2 resulting from the third drive signal S3 by inversion by an inverter.The breakdown voltage of the voltage limiting circuit SB is therebyreduced only when the third transistor T3 is activated.

Whereas in the exemplary embodiment in accordance with FIG. 5 thebreakdown voltage of the voltage limiting circuit SB is dependent on thetemperature in the region of the first transistor, FIG. 6 shows afurther exemplary embodiment of the circuit configuration according tothe invention, in which the drive circuit A3 of the third transistor T3and the drive circuit A2 of the second transistor T2 are fed a signal S6dependent on the current through the transistor T1. The signal S6 isgenerated by a current sensor IS connected in series with the firsttransistor T1.

I claim:
 1. A circuit configuration for driving a load, comprising: afirst connecting terminal for connecting to the load; a secondconnecting terminal; a first drive input for receiving a first drivesignal; a semiconductor switching element having a first load terminalconnected to said first connecting terminal, a second load terminalconnected to said second connecting terminal, and a drive terminalcoupled to said first drive input; and a voltage limiting circuitconnected between said first load terminal and said drive terminal ofsaid semiconductor switching element, said voltage limiting circuithaving a second drive input receiving a second drive signal, saidvoltage limiting circuit having a threshold voltage being dependent onthe second drive signal, said voltage limiting circuit having a seriescircuit containing a first voltage limiting element; and a secondvoltage limiting element, one of said first and second voltage limitingelements being selectively short-circuited in response to the seconddrive signal; and the second drive signal being dependent on atemperature in a region of said semiconductor switching element.
 2. Thecircuit configuration according to claim 1, wherein at least one of saidfirst and second voltage limiting elements is a zener diode.
 3. Thecircuit configuration according to claim 1, including a furthersemiconductor switching element connected in parallel with said secondvoltage limiting element and said further semiconductor switchingelement having a drive terminal receiving the second drive signal. 4.The circuit configuration according to claim 1, wherein the second drivesignal is dependent on a switching state of said semiconductor switchingelement.
 5. The circuit configuration according to claim 4, wherein thesecond drive signal is dependent on the first drive signal.
 6. Thecircuit configuration according to claim 5, including a drive circuitconnected to said second drive input of said voltage limiting circuit,said drive circuit having an input and an output, the second drivesignal being available at said output and a temperature signal beingpresent at an input of said drive circuit.
 7. The circuit configurationaccording to claim 3, including another semiconductor switching elementconnected between said drive terminal and said second load terminal ofsaid semiconductor switching element, and said another semiconductorswitching element having a drive terminal.
 8. The circuit configurationaccording to claim 7, including a drive circuit connected to said driveterminal of said another semiconductor switching element.
 9. The circuitconfiguration according to claim 8, wherein said drive circuit is fed atemperature signal that is dependent on a temperature of saidsemiconductor switching element.
 10. The circuit configuration accordingto claim 8, wherein said drive circuit is fed a current signal dependenton a current through said semiconductor switching element.
 11. Thecircuit configuration according to claim 7, wherein said semiconductorswitching element, said further semiconductor switching element and saidanother semiconductor switching element are field-effect transistors.